The general elimination of landfill sites and the increase in landfill costs have created a significant problem in industrial nations for waste and rubbish disposal and have increased the demand for systems which reduce the landfill volume requirements for such disposal.
In the paper industry, there is a problem with the disposal of paper sludges or slurries which have no better use and which in the past have had to be incinerated with tree bark or other fuels. Even in such cases, ash is produced which must be dumped in landfills. With time, the costs of this disposal have increased and the problem of available landfill sites has affected the disposal of such ash as well.
Paper sludge or slurry may have up to about 50% water, the balance being short cellulose fibers and so-called fillers The fillers which are used in the paper industry are natural mineral materials like kaolin and calcium carbonate.
In the production of cement clinker utilizing conventional dry process techniques, i.e. rotary cement kilns (see, for example Chemical and Process Technology Encyclopedia, Considine, McGraw Hill, Inc., New York, 1974, pages 237 ff), it is known to produce a secondary combustion at the rotary kiln inlet utilizing low-grade fuels such as rubber tire scrap, coal-containing residues, oil sludges or the like to cover a portion of the overall energy requirement for the process and thus reduce the energy for the primary firing.
It should be evident that fuels which have a high moisture content, for example, substances which are so moist that the evaporation energy for driving off the moisture will exceed the contribution to the combustion energy of the fuel substances within the composition, will be of little value in such process since there is little if any energy saving through the use of such compositions. For example, clarifier sludges can only be used as fuels in a practical way if they are dried utilizing an external energy source, stored and then introduced into the cement-making process. Such systems have the drawback that there may not be any overall energy saving although the process does allow the destruction of a waste which might otherwise occupy a landfill.
In British patent 251,558 it has been proposed to utilize exhaust gases from the oven or kiln for the drying of low-grade fuels. It is also known to introduce the ash produced by the drying and incineration of wastes into a cement-making process.